The invention relates to a method of producing ornamental link chains (Venetian chains).
In general there are two requirements for ornamental link chains: they must be esthetically appealing and at the same time durable. The esthetic effect depends on the surface nature and the uniformity of the chain links. For Venetian chains, this means that with a precisely rectangular shape of the chain link, its visible surfaces must be completely plane. Moreover, since the human eye detects the slightest deviations in the arrangement of the chain formation, the internal dimensions must be precisely adhered to. Even minor deviations in the transverse play between interengaging chain links detract appreciably from the esthetic effect. The transverse play can be unified subsequently within limits by an intensified striking operation to which the chain is subjected. If the longitudinal arms of the chain links are rounded beyond the longitudinal axis, for example as a result of inadequate tool condition, so that the individual chain link deviates from the desired precise rectangular shape, planeness and parallelism between visible surfaces remote from one another, such condition can be achieved subsequently to a limited extent by a diamond operation which, in comparison with a common process to achieve a desired surface polish, is directed to a greater removal of material. Intensified hammering and shape-correcting diamond treatment as additional correctional measures increase the cost of the piece in a disadvantageous manner. The shape-correcting diamond treatment further leads--particularly with noble metals--not only to an appreciable loss of material by shaving waste, but also to the weakening of the thickness of the wire of the particular chain link, as a result of which the durability of the chain in question is adversely affected.
As already mentioned, a precise production depends to a great extent on the condition of the tools. The tools must be able to produce possibly extremely small dimensions of the flat wire and chain link. The tools are usually very very small and are subject to severe wear--even when being made out of a high quality material. Consequently, they have to be frequently reconditioned and replaced. This leads, in a disadvantageous manner, not only to high tool costs but also to inevitable down-time of the machines in question. In view of the necessary mass production to cover a large output demand, high costs of individual pieces cannot be tolerated.
According to the prior art, a process for the prefabrication of the chain is followed by a process for bringing the chain into a condition ready for soldering. The Venetian chain is first prefabricated on a first apparatus and then is transferred to a second apparatus for carrying out a rhythmic striking operation for the preset adjustment of the minimum width of a butt-joint gap to receive solder between ends of the individual links to be connected by soldering. The separation of the manufacturing process in time and also in place because of the necessity for two apparatuses, proves expensive in a disadvantageous manner as is obvious.
Even the method of prefabricating the chain has disadvantages. Such method is explained immediately below in connection with an apparatus designated FVZ as well as a stamping apparatus designated FSCHL which are described in a catalog published by the assignee of this application in 1974 all of which are regarded as prior art by the Applicant.
A flat wire is pulled in horizontally against a stop and cut to a predetermined length by lowering a cutting tool. A preliminary bending tool with one of its outside edges serves as a cutting tool. It is detachably secured in a tool holder which is adapted for travel up and down along a vertical guide axis transversely to the pulling-in direction of the flat wire. After cutting the wire to length, the section of flat wire is bent into a U over a pre-bending mandrel by means of the preliminary bending tool. The pre-bending mandrel has a rectangular cross-section. Its width corresponds to the sum of the lengths of the two axial arms and the first transverse web. As a result of the springing back of the two U-arms, an adequate surface friction results between the surface regions of the section of flat wire and the immediately adjacent surfaces of the preliminary bending tool. Thus the U is firmly held in the preliminary bending tool.
The pre-bending mandrel is now removed axially from the range of action of the preliminary bending tool, transversely to the feed direction of the flat wire, and a closing mandrel is freed. The width of the closing mandrel corresponds to the length of the first transverse web and consequently one side of the rectangle to be described by the individual link. A finish bending tool for closing the U to form the individual link is disposed above the preliminary bending tool, so that the second bending operation (closing) is completed by further lowering of the holder. In the course of this, the second bending mandrel rests on an upper end of a pair of holding pincers. The latter includes two gripping elements with adjacent contact surfaces disposed vertically. These have an upper terminal edge with a horizontal surface forming the upper end of the holding pincers in common. The gripping elements are adapted for deflection out of a basic position in order to increase a lateral spacing between the two contact surfaces. In the basic position, the spacing between the contact surfaces corresponds at least to the width of the flat wire. If the holder is now lowered further to complete the second bending operation, then the two axial arms are formed by bending at right angles in the transverse region of the U, the short arms of which approach one another with their free ends over a curved path until they form a mutual butt joint. The individual link closed in the manner described above now surrounds the closing mandrel, over its periphery in a region defined by the basics-position spacing between the two contact surfaces.
(If a plurality of individual links have already been prefabricated then the section of chain formed by them hangs with the last individual link closed on the second bending mandrel). A vertical central axis of the holding pincers is in alignment with the guide axis of the tool holder. The holding pincers are now turned, for example in a clockwise direction, through 90.degree. about the central axis out of their supporting position for the second bending mandrel and the latter is lowered to a preset height between the contact surfaces. The gripping elements now take over the closed individual link in that they approach one another with their contact surfaces until they come into friction contact with the outer surface in question of the axial arm. The first transverse web of the individual link lies adjusted in an assembly plane. The second bending mandrel is displaced axially and is pulled out of the individual link. In order that the two short U-arms of the next individual link (they each form half of the second transverse web) may engage in the individual link adjusted in height, and can form a butt joint with their two ends on closing, the holding pincers are turned through 90.degree. in counterclockwise direction and so again into their supporting position for the second bending mandrel during the second bending operation for finish bending (closing) the following individual link. The next intake of flat wire can only be effected when the tool holder has returned vertically upwards into its initial position for the cutting to length.
In the above-described method of prefabrication it proves a disadvantage that the method steps of cutting to length, preliminary bending and closing take place at different heights of a region extending round the vertical guide axis of the tool holder. As a result, this region is blocked for the duration of the method steps to be carried out on an individual link. It is a further disadvantage that the preliminary bending tool serves at the same time as a cutting tool for cutting to length the flat wire fed in and consequently is subjected to increased wear at one side. Because the closing tool is disposed rigidly immediately above the preliminary bending tool, no holding down means can be used during the preliminary bending for the region remaining between the two short U-arms. Consequently, the flat wire can slide round the two bending edges of the bending mandrel, and thus it is not stretched horizontally but forms an arc. As a result, the individual part is rounded in the region in question.
From the combined arrangement of preliminary bending and closing tool it follows that the individual parts must be very thin, which is appreciably at the expense of the bending strength and so frequently leads to irregularities in the individual link or to tool breakage. Since the parts in question must be cold tough, they are frequently produced individually by hand work from bandsaw steel, particularly at the cutting edge side of the preliminary bending tool. This proves extremely time-consuming and costly. It is a further disadvantage that the holding pincers have to be turned twice through 90.degree. for each individual link, in the course of which the gripping elements must be controlled for their movement. According to the prior art, each method step for controlling the drive or actuating the individual components (feed device, tool holder, bending mandrels, holding pincers, etc.) has a preset section disposed on a cam. The cam usually extends, closed in itself, along a guide edge, for example the periphery, of at least one disc driven in rotation. As a result of the need for idle steps which are not directly effective in the process, the course of the process is extended in time and an additional expenditure for apparatus is necessary.
Finally, as already mentioned, it is a disadvantage that in order to achieve the state of the chain in which it is suitable for soldering preparation, a further method step has to be carried out on a second apparatus to adjust the width of the gap of the butt joint. After the closing of the gap the two axial arms of the individual link can actually spring back. As a result, the width of the butt-joint gap cannot be adequately controlled, which may lead to rejects in the soldering of the chain. In addition, the parallelism of the outer surfaces is not assured. A springing back of the two axial arms is to be expected to a disturbing extent, particularly if the flat-wire material is comparatively resistant to bending. This is particularly the case with alloys with a low proportion of noble metal (for example gold). This difficulty becomes obvious with solid wire, particularly of small dimensions, so that costly soldering wire had to be used, which in turn requires special soldering to be used for venetian chains.